The present invention relates generally to coal gasification systems and more particularly to an in situ coal gasification system wherein a gas with optimum BTU content can be recovered.
There are many deposits in the coal regions of the world that are favorably situated, but are commercially unminable due to the high sulfur content of the coal, the deposit itself is a prolific aquifer, the deposit is gas prone, or the like.
While high sulfur content of the coal presents no unusual hazards to manpower underground, burning of the coal above ground results in unacceptable pollution of the atmosphere due to emissions of sulfur dioxide (SO.sub.2), sulfur trioxide (SO.sub.3) and gaseous sulfuric acid. Removal of the sulfur from raw coal is a costly undertaking, the costs generally exceeding the market value of the residual coal. In the coal deposits where the deposit itself is an aquifer, dewatering is a costly and continuing undertaking that is compounded by disposal problems of contaminated water. Coal deposits that are gas prone contain ever present perils to manpower underground such as the hazards of fire and explosion and unsafe breathing atmospheres.
In burning coal above ground as a fuel, one attempts to attain a maximum practical calorific value from the coal. In so doing the hydrogen content is burned to water vapor and the carbon content is burned to carbon dioxide (CO.sub.2). Reasonable attempts are made to prevent the escape of free hydrogen and carbon monoxide (CO) into the flue gases because hydrogen has a heat content of 320 BTU per standard cubic foot and carbon monoxide has a heat content of 315 BTU per standard cubic foot. Escape of these gases unburned represents a significant loss in efficiency, and the environmental impact of releasing large quantities of carbon monoxide into the atmosphere presents unacceptable hazards. Thus, the hearth, furnace, combustion chamber and the like for coal are kept in an oxidizing environment, so that all gases will be essentially fully oxidized before being discharged into the atmosphere.
All coals contain sulfur, varying from less than one percent to ten percent or higher. When coal is burned in an oxidizing environment its sulfur content is largely burned to sulfur dioxide which is a reasonably stable compound. Sulfur dioxide, however, may be further oxidized in the presence of a catalyst, for example, iron into sulfur trioxide which is an unstable compound. Most combustion chambers have iron components, which serve as a mild catalyst to generate sulfur trioxide in the exit gases. In the same exit gas there is water vapor resulting from the combustion of hydrogen. Unstable sulfur trioxide readily combines with water vapor to form gaseous sulfuric acid (H.sub.2 SO.sub.4) in the exit gases. These sulfur products although representing a small percentage of the exit gases, produce significant dileterious affects on animal and plant life when introduced into the atmosphere. Even with small percentages, the volumes of sulfur products can be enormous. It is for these reasons that governmental agencies have increasingly placed more stringent requirements on maximum allowable sulfur levels in fuels.
Repeated attempts have been made to develop suitable means to remove sulfur dioxide and sulfur trioxide from stack gases. A satisfactory method has not been found to reduce the sulfur content of raw coal to desired levels. To meet governmental imposed environmental standards, the coal industry has been forced to go to deposits with lower sulfur content, and to bypass vast deposits of higher sulphur coals. In the United States, the low sulfur coals tend to be at great distances from population centers and the points of use for the coal. Further, low sulfur coals tend to be high in moisture and ash contents, thus resulting in lower BTU values per pound. Transportation costs, therefore, tend to become a disproportionate part of the cost of BTUs at the point of use.
It is apparent, therefore, that a new system is desired to permit the use of high sulfur coals particularly those that are favorably situated in regard to points of use. It is an object of this invention to introduce such a system.